This invention relates to a structure for providing internal combustion engine noise suppression.
Typical marine engines are noisy, especially when being operated at higher rpm""s while driving a vessel rapidly through the water. This noisy operation is extremely unattractive to occupants of the vessel, as well as to passers-by, and it is highly desirable to reduce this noise without reducing vessel efficiency. Further, regulatory bodies, in their desire to improve the environment, are imposing emission standards on marine vessels. These standards not only regulate the contents of the emissions but also apply to the noise level of the emission. It is therefore highly desirable to provide a marine engine that is noise reduction efficient without detracting from the vessel operating efficiently.
More general than the noise reduction is noise control. Noise control requires an understanding of the vibro-acoustic behavior of the article in question with its environment. If boundary conditions permit, approximations can be made by isolating the article from its environment. This cannot be done xe2x80x9csimplyxe2x80x9d for an integrated structure. For example, an outboard marine engine is an integrated structure. To capture correctly the vibro-acoustic behavior of an outboard engine, the engine should be fully assembled, mounted to a boat and in the open water. For example, feedback from the added inertia of the water as the boat travels in the water could produce a narrow-band spectrum different from a steady-state condition. There is also feedback from the components of the engine, for example, the crankshaft and block can produce a phenomenon that does not exist for either part acting alone.
To determine the acoustic xe2x80x9cfingerprintxe2x80x9d for an integrated structure such as an outboard marine engine, a narrow-band analysis must be performed. This will allow identification of tones, i.e., frequency responses, of the interacting components. The components corresponding to these responses can be identified from the frequencies, i.e., based on wavelength and speed of sound. Vibro-acoustic treatments can be designed and or critically placed to attenuate or simply move a tone from one frequency to another. The effectiveness of this effort is based on the precision of the data and the methodology by which the data is acquired.
The precision of the data is a function of the frequencies of the data collected and of the transducer sensitivity. The frequency range of interest is a function of human hearing, i.e., 10 kHz is sufficient. For the present work, data was collected using accelerometers and microphones. Accelerometer data was collected to 5 kHz at 1 Hz bandwidth; microphone data was collected to 10 kHz at 2.5 Hz bandwidth. Acoustic intensity testing and stethoscopic probing both showed agreement that over 80% of the vibro-acoustic energy produced by a particular outboard marine engine was coming from below the interface between the engine""s upper and lower motor covers.
Thus there is a need for a structure which can be incorporated inside an outboard marine engine to achieve noise suppression without adversely impacting engine performance.
The present invention is directed to an improved engine having means for controlling and reducing the noise emitted by the engine. Although the preferred embodiment is disclosed in the context of an outboard marine engine, persons skilled in the art will readily appreciate that the means for noise suppression could also be installed inside the housing for the powerhead of an inboard marine engine or any other type of powerhead encased in a housing. (The terms xe2x80x9cpowerheadxe2x80x9d and xe2x80x9cmotorxe2x80x9d will be used interchangeably throughout the written description and the claims.)
One approach for reducing the vibro-acoustic energy produced by an engine is to shroud the powerhead with a blanket of material that both damps vibrations and blocks/absorbs acoustic wave energy. Such material will be hereinafter referred to as a xe2x80x9cvibro-acoustic treatmentxe2x80x9d. The vibro-acoustic treatment in accordance with the preferred embodiments is applied on the inner surface or surfaces of a motor housing of a propulsion system.
In accordance with the preferred embodiment of the invention, the noise generated by an outboard marine engine can be controlled and reduced by installing vibro-acoustic treatments inside both the upper and lower motor covers, forming a shroud around the powerhead. Each treatment comprises an acoustic barrier laminated to an open-cell foam core that absorbs acoustic energy. The mass per unit area of the acoustic barrier for the lower motor cover is preferably greater than that of the acoustic barrier for the upper cover, while the foam layer for the lower motor cover is preferably thicker than the foam layer for the upper motor cover. These vibro-acoustic treatments are designed to work together. For example, the treated lower motor cover is designed to attenuate the overall acoustic energy and the treated upper motor cover is designed to shape the frequency spectrum, i.e., to act as a filter. This is analogous to a home stereo system, where the lower motor cover is the power amplifier and the upper motor cover is the equalization filter. The treated upper motor cover needs the treated lower motor cover in order to perform as it was designed to. The treated lower motor cover is independent of the treated upper motor cover, although the treated upper motor cover provides the refined sound enabling a superior sound quality. The treated lower motor cover provides the noise reduction, while the treated upper motor cover, in concert with the treated lower motor cover, provides the sound quality, i.e., the noise control.
More specifically, the primary purpose of the treatment in the upper motor cover is to tailor the narrow-band acoustic spectrum so that it is xe2x80x9cpleasingxe2x80x9d to the human ear. The human ear is most sensitive to frequencies between 1,000 and 3,000 hertz. In accordance with the preferred embodiment, the vibro-acoustic treatment inside the upper motor cover is designed to shift acoustic energy in the frequencies between 1000 and 3000 Hz to frequencies below 1000 Hz and above 3000 Hz. In contrast, the lower motor cover can be considered the primary receiver of the structure-borne noise and vibration. To best attenuate this energy, the vibro-acoustic treatment for the lower motor cover is designed to attenuate across a wide frequency range (e.g., 0 to 4,000 hertz), but was optimized for the frequencies under 1000 Hz. The vibro-acoustic treatment on the upper motor cover was not designed to attenuate frequencies below 1,000 hertz because to do so would require additional mass inside the upper motor cover, which additional mass would negatively impact overall engine performance, e.g., by interfering with the intake of air.